1. Field of the Invention
The present invention generally relates to the field of mass spectrometry and, more particularly, to an electrooptical detector for use at the focal plane of a mass spectrometer.
2. Description of the Prior Art
The term mass spectroscope or mass spectrometer, generally designated as an MS, is applied to a device which has the ability to separate ions according to their mass-to-charge ratios. Typically, in an MS, ions from an ion source are made to pass to a mass analyzer, consisting of an electrostatic analyzer and a magnetic analyzer. Therein they are separated into the appropriate mass-charge groups. This is achieved by the application of electric and magnetic fields. The separated ion groups in the form of separate ion beams are directed to a detector wherein they provide an indication of their presence.
Some MS's are of the type in which different ion groups are detected simultaneously. In these devices the different ion beams are brought to sharp focus at a focal plane which is at or near the exit boundary of the magnetic analyzer, at which a strong magnetic field is present. Typically, the detector is a photographic plate or simply a photoplate which is placed at the focal plane and on which the separated focused ion beams produce images.
One of the earliest and well known MS of this type is the Mattauch-Herzog MS. Although the photoplate is very suited for simultaneously detecting many ion groups or species over a wide mass range (36:1), such as 28-100 amu) with good resolution (&gt;100 lines/mm) its utility is restricted for the following reasons. The photoplate sensitivity is low, requiring about 10.sup.3 to 10.sup.4 ions to produce a measurable line image. Also the plate's dynamic range is typically less than 30:1. In addition the time of recording is often relatively long, the plate need be inserted and removed from the evacuated chamber of the MS. Also, after exposure, it has to be developed, as a conventional photographic negative, and thereafter a densitometer must be used to analyze the exposed plate.
In order to overcome the disadvantages of the photoplate as the detector, electronic detection systems have been developed. These electronic detectors are designed to increase the detection sensitivity and/or shorten the time required to collect both qualitative and quantitative analytical results and simplify the associated data reduction problems. To date these devices have only been used in MSs of the scanning type in which different ion groups are successively swept through a single slit by varying the electrostatic or magnetic field. The slit is located at a focal point which is typically remote from and, therefore, not within the magnetic field. As the electrostatic or magnetic field is varied different ion groups are bent so that their beams successively exit through the slit. At any time only a single beam exits the slit. Nier's double-focusing MS is typical of such an MS, as described in "Mass Spectrometry" by Charles A. McDowell, Library of Congress Catalog Card Number 62-22201.
The electronic detector typically consists of an electron multiplier. Ions which pass through the slit enter the electron multiplier and produce secondary electrons which are then detected to indicate the presence of the ion beam at the slit. In spite of the inherent high sensitivity of the electron multiplier, which is capable of producing on the order of 10.sup.3 -10.sup.8 electrons per ion, and the popularity of the electronic detector it also has a number of severe limitations.
The most important of these is a loss of sensitivity by 4 to 6 orders of magnitude due to the limited observation time for the sample as a whole, as well as each ionic group of species of the spectrum. Since the sample pressure in an ion source may vary rapidly with time (as it emerges from a solid probe, a gas chromatograph interface, etc.) only about 1/10th of the sample passing through the ion source can be used to obtain a representative spectrum (scanned in a time short enough for concentration changes not to occur) and only 1 ion in 10.sup.3 -10.sup.5 is collected as one scans a typical spectrum. Consequently, today's electronic detection methods in the scanning mode are no more sensitive than the old photographic methods. The awkwardness of processing the photographic plate has been replaced by the requirements of using special techniques in initiating scans at the proper time where spectra of small transient samples are to be recorded.
Recently, an electro-optical detection system has been proposed for use in a magnetic sector MS, in which the mass range of the ion groups which are simultaneously detected is very narrow, generally in the order of not more than .+-.10%, or a mass range of 1.1:1. In such a magnetic sector MS the focal plane is also outside the magnetic field of the MS analyzer. The proposed electro-optical detector includes a micro-channel electron multiplier array (hereinafter defined as MCA) which is used as a primary ion-electron converter. The MCA is located at the focal plane which is outside the magnetic field. It is electro-optically coupled to a high resolution metallized phosphored screen or plate which is deposited onto a fiber optical coupler to a vidicon camera tube.
The proposed electro-optical detector operates quite satisfactorily in the magnetic sector MS, thereby eliminating the problems inherent in the use of the photoplate as the detector. However, the magnetic sector MS is of limited application since it can only be used to detect simultaneously ion groups over a very narrow mass range.
Attempts to adapt the above-described electro-optical detector for use in a wide mass range MS such as the Mattauch-Herzog MS by placing the MCA at the MS's focal plane, whereat the photoplate is conventionally placed, have failed. The failure is due to the fact that at the focal plane of a Mattauch-Herzog type MS strong transverse magnetic fields are present. These, combined with an orthogonal electrostatic field needed to accelerate the secondary electrons to the phosphor plate inhibit the electrons from ever reaching the phosphor plate to produce an image thereat. The electrons which exit the MCA in the presence of the strong magnetic field undergo a cycloidal motion between the MCA and the phosphored plate, never reaching the latter, unless the magnetic field of the MS is greatly reduced, at the price of reduced mass range operability. Thus, a need exists for an improved electro-optical detector for use in an MS capable of simultaneously detecting many ion species over a wide mass range, such as the Mattauch-Herzog MS.